1. Technical Field
The present invention relates to a Coriolis flowmeter that is used to obtain a mass flow rate and/or density of a measurement fluid by detecting a phase difference and/or vibration frequency proportional to Coriolis forces acting on a flow tube.
2. Background Art
In a Coriolis flowmeter, a tube through which a fluid to be measured flows is supported at one end or both ends thereof, and vibration is applied to a portion of the tube around the supporting point in a direction vertical to the flowing direction of the tube (hereinafter, a tube to which vibration is applied is referred to as a flow tube). The Coriolis flowmeter is amass flowmeter, which utilizes the fact that the Coriolis forces applied to the flow tube when vibration is thus applied thereto, are proportional to a mass flow rate. The Coriolis flowmeters are well known, and the flow tube structures thereof are roughly classified into two types: a straight tube type and a bent tube type.
In a Coriolis flowmeter of the straight tube type, when vibration is applied to a straight tube, whose both ends are supported, in a direction vertical to the straight-tube center portion axis, a difference in displacement of the straight tube due to the Coriolis forces is generated between the support portions and the central portion of the straight tube. That is, a phase difference signal is obtained, and, based on this phase difference signal, the mass flow rate is detected. The straight tube type Coriolis flowmeter thus constructed has a simple, compact, and solid structure. However, there still has a problem in that it is impossible to achieve high detection sensitivity.
In contrast, the bent tube type Coriolis flowmeter is superior to the straight tube type Coriolis flowmeter in that it allows selection of a shape for effectively obtaining the Coriolis forces. In fact, it is capable of performing mass flow rate detection with high sensitivity. Known examples of the bent tube type Coriolis flowmeter include one equipped with a single flow tube (see, for example, JP 04-055250 B), one equipped with two flow tubes arranged in parallel (see, for example, Japanese Patent No. 2939242), and one equipped with a single flow tube in a looped state (see, for example, JP 05-069453 B).
Incidentally, as a driving means for driving the flow tube, a combination of a coil and a magnet is generally employed. Regarding the mounting of the coil and the magnet, it is desirable to mount them at positions not offset with respect to the vibrating direction of the flow tube. In view of this, as disclosed in Japanese Patent No. 2939242 two flow tubes arranged in parallel are mounted so as to hold a coil and a magnet between them. Thus, a design is adopted in which the distance between the two flow tubes opposed to each other is at least large enough to enable the coil and the magnet to be held therebetween.
In the case of a Coriolis flowmeter in which two flow tubes respectively exist in planes parallel to each other and which exhibits a large caliber or high flow tube rigidity, it is necessary to enhance the power of the driving means, so that it is necessary to hold a large driving means between the two flow tubes. Thus, a design is adopted in which the distance between the flow tubes is of necessity large even at the fixing end portions constituting the roots of the flow tubes.
However, an increase in the above-mentioned distance at the fixing end portions involves the following problem: it leads to a deficiency in rigidity at the fixing end portions, with the result that vibration leakage is likely to occur (the flow tubes vibrate owing to bending vibration, and then the vibration leaks out).
On the other hand, in the construction as disclosed in JP 05-69453 B, in which a single flow tube is looped, another problem is involved: as shown in FIGS. 11 and 12, it is necessary for a bent tube portion 101 to exist between a first curved tube portion 102 and a second curved tube portion 103. Because of that necessity, the quick manufacture of the bends shown in the figure is rather difficult and, further, there is a problem in terms of the resistance to pressure of the tubes.